This invention relates to a meter for producing, detecting and registering magnetic changes occurring in a material, such as steel, with temperature changes.
The great significance of steel as structural material is largely based on the possibilities of widely effecting its mechanical properties by means of heat treatments. In order to achieve a desired result a suitable steel, as to the decomposition properties of austenite, must be chosen at the same time considering available cooling possibilities.
The microstructure resulting from decomposition of austenite in steel, upon which microstructure the properties of steel depend, is normally presented as isothermal or continuous cooling phase transformation diagrams (IT- and CCT-diagrams) of which the latter has primarily practical significance.
Several methods are currently known for determination of such diagrams. Many of the methods used are only suitable for isothermal phase transformation studies. In principle, at least the following methods are known to be suitable for studies of continuous cooling: X-ray diffraction, resistance change measuring, dilatometer, magnetic measuring, thermic analysis.
On the basis of all these methods there have been efforts made to produce phase transformation meters but there have been such difficulties involved with measuring technique and equipment that only on the basis of the dilatometer method it has been possible to develop sets of equipment working at least satisfactorily. Thus, all phase transformation meters suitable for general use are dilatometers at the present time.
In the dilatometer method the detection of phase transformations is based on the detection of volumetric changes associated with these transformations. The test sample consists of a thin cylindrical rod whose alterations of length are transmitted by means of a quartz rod touching mechanically the end of the test rod to an inductive detector. Difficulties in this method are caused by the great measuring sensitivity required by small alterations of length and also a mechanical contact required between the measuring detector and the test rod. Therefore dilatometer equipment is not at all well suitable for phase transformation measurements occurring at high cooling rates (&gt; 100.degree. C/s).
The magnetic changes associated with phase transformations have been attempted to be utilized when developing a phase transformation meter. This approach has been presented e.g. in Steel, January 1968, page 14. This device consists of two pairs of coils, one of them constituting a comparison pair. The coils are adjusted by means of bridge connections so that no current passes through the meter between the bridge connections. Now, when the magnetic properties of the test material change, they produce current between the coils, its magnitude being readable from the current meter and magnitude of the magnetic change occurred can be concluded therefrom. Like other similar equipment this set has not attained sufficient reliability, either. In addition, the use of two pairs of coils and quite a few bridge connections is cumbersome, as the current value obtained must nevertheless be transformed into estimated magnitude of phase transformation.
A device having commercial significance, in order to be superior as compared to equipment functioning according to the dilatometer principle, must comprise the following properties:
the device must be suitable for phase transformation measurements also at high cooling rates (.about.100.degree. C/s down to 1.degree. C/min) PA1 measuring results are unambiguous and easily estimable PA1 the device is simply and easy operated PA1 the test sample is simple and inexpensive PA1 efficient life of the device is long PA1 it must be possible to simulate various practical cases and theoretical applications by means of the device.